Barnard’s Star and the Detection of Extrasolar Planets
Barnard's Star:
RA = 17h 57' 50" Dec = +4° 38' 19" Red dwarf in Ophiuchus
Distance = 1.81 parsecs, m = +9.5, Proper Motion = 10.31"/yr
May 1993 May 2004
Barnard's Star. Field size 15 x 15 arc
minutes. Barnard's Star has moved north by 113.6" (nearly 2 arc minutes - more
than twice the angular size of Jupiter) in the 11 year baseline in these photos.
Photos by Richard Nugent.
In the September 15, 1916 issue of The Astronomical Journal
and the September 7, 1916 issue of Nature , an article appeared that dealt with the discovery of a
rather small, insignificant star that demonstrated a large proper motion. The
purpose of the article was to alert the astronomical world that indeed, E.E.
Barnard detected a unique find, that is a star with a proper motion larger than
any star that had been studied previously. The large proper motion was
calculated by Barnard to be approximately 10.3 arcseconds per year. The proper motion is defined as "the apparent
angular motion per year of a star on the celestial sphere, i.e., in a direction
perpendicular to the line of sight" . Proper
motion is attributed to two basic premises; the star can move of its own accord
and the star's galaxy can also move. Because of the vast distances between us
and stars, the stars appear to be glued in the night sky and that the only
apparent movement of stars that we see is due to the rotation of the Earth about
its axis. If you look at a constellation, such as Orion this year, you will note
that it will look exactly the same next year, and the year after that, and so
on. It takes thousands of years to see that the stars have moved by seeing a
change in the position of the stars with respect to one another. For example,
the stars in the Big Dipper did not have the same positions 2,000 years ago, and
hence, did not look like the Big Dipper.
Because of the large proper motion of Barnard's Star, astronomers felt the
need to determine additional information regarding this amazing star. It was
found to be a red dwarf, and its distance from the Earth to be 1.82 parsecs
(5.95 light years); practically a next door neighbor in astronomical terms. Not
only was the distance determined but the star was found to be moving closer to
us. By 11,800 AD it will be as close as 1.16 parsecs (3.8 light years) from us. Because Barnard's Star is a red dwarf (common in our
galaxy), its close proximity to us, and its large proper motion, this star was a
prime candidate for further study regarding the search for extrasolar planets.
It should be noted, however, that it was the large proper motion of the star
that encouraged astronomers to determine its other physical parameters.
Before the story of Barnard's Star continues it would be best to consider a few astronomical terms that will play a large role in the supposed confirmation of planets revolving around the star. These terms are and perturbation. Astrometry is that part of astronomy that measures the proper motion of stars as a function of time. In the case of Barnard's Star, photographic plates were examined over many decades in order to determine its motion. Perturbation, on the other hand, is a way of describing any abnormalities in the stars motion. Does the star move in a straight line or does it exhibit some wobble (or sinusoidal) motion? If so, this wobble could be explained by gravitational forces between the star and an unseen body or bodies revolving around the star.
With the above in mind, the story of this star continues with the photographic plates of Peter van de Kamp. Working at the Sproul Observatory of Swarthmore College, van de Kamp devoted most of his life over 2,000 plates of Barnard's Star that he and his students had taken from 1938 through 1962. According to van de Kamp, a wobble in the movement of Barnard's Star was detected which he determined was the result of a body revolving around the star. The body, according to van de Kamp, was 1.6 times the mass of Jupiter and that its rate of revolution was 24 years. He also suggested that the orbit was elongated.
Over the years Peter van de Kamp published a series of papers refining his initial results of the companion planet revolving around Barnard's Star. In the March 1969 issue of the Astronomical Journal he reconsidered a number of physical parameters regarding the companion. From photographic plates from 1916-1919 and a large number of plates from 1938-1967 he determined that the planet (van de Kamp sometimes referred to it as "his" planet) revolved around Barnard's Star every 25 years and that it was extremely large, i.e., 1.7 times the mass of Jupiter. In August of the same year another article was published by van de Kamp in the Astronomical Journal stating that while reconsidering the photographic plates in his possession, there were not one but two planets revolving around Barnard's Star. Not only were there two planets but instead of an elongated orbit, the orbits of both companions were found to be circular. In addition, by reconsidering his calculations, he ascertained that one of the planets revolved around the star every 26 years while the other 12 years. Likewise, the previous mass he had given the first companion at 1.7 times the mass of Jupiter was incorrect. At this point in time the companions seemed to have masses 1.1 and 0.8 the mass of Jupiter. In 1975, van de Kamp published yet another article analyzing astrometric data from 1950 to 1974. According to this paper the masses of the two planets were determined to be 0.4 Jupiter masses and 1.0 Jupiter masses. In addition, the number of years that the planets revolved around the star were recalculated to 22 years and 11.5 years. The 0.4 Jovian mass planet was determined to be the body that revolved around the Star every 22 years while the 1.0 Jovian mass planet was thought to revolve around Barnard's Star every 11.5 years.
However, van de Kamp did not stop there. He continued to amass more photographic plates on Barnard's Star. In 1982, with measurements of photographic plates from 1938 through 1981, he published yet another paper with slightly different results of the planetary companions that had been published previously. In his 1982 paper , he reconfirmed the fact that both orbits were circular. However, he reconsidered their periods of revolution around Barnard's Star as well as the planets' masses. In re-examining his data, he reconciled the planets revolutions around the star to be 12 and 20 years with masses of 0.7 and 0.5 the size of Jupiter. According to van de Kamp the reason for the changes in the physical parameters of the two planets had to do in part with the reference stars he was using when examining the photographic plates. In additon, he now had more years of photographic plates to work with than what he had when publishing his earlier papers.
While van de Kamp was honing his measurements on the two companions revolving around Barnard's Star, a few papers in 1973 were published that questioned his claims either directly or indirectly. Astrometrists Gatewood and Eichhorn examined photographic plates taken with a 20-inch refractor at the Van Vleck Observatory located at Wesleyan University and the 30-inch Thaw refractor of the Allegheny Observatory in Pittsburg. They were unable to detect any wobble in the proper motion of Barnard's Star. In short, if there was no wobble, then no planetary companions existed. In the same year, John L. Hershey, while working at the Sproul Observatory, analyzed the same photographic plates as van de Kamp. He made a systematic study of not only Barnard's Star but a number of other stars found on the plates. A total of twelve stars were considered in the study. It should be noted that these plates were a result of the use of the telescope at the Sproul Observatory. To Hershey's amazement, not only did he detect a wobble in the proper motion of Barnard's Star, but wobbles in all the stars compiled in the study. A number of inferences could be made with these results. One possibility is that all the stars had planetary companions, or on the other hand, there were problems with the Sproul telescope that had been used to photograph the stars in question. Indeed, if it were the latter case, then van de Kamp, who based his conclusions on the photographs was using incorrect data, as was Hershey. It was found that there seemed to be a large discrepancy in the results of star movement on the plates in 1949 through 1956 and then again in 1957. In 1949 a new cell for the 24 inch lens was installed and in 1957 the objective lens was adjusted. (13) Could the readjustment of the telescope be the reason for the big jump in star positions?
Years later, Robert Harrington, using the 61-inch reflector at the United States Naval Observatory at Flagstaff, compiled over 400 plates of Barnard's Star. Unfortunately, in studying these plates Harrington could not detect a wobble. Laurence Fredrick, working with a 26-inch refractor at the Mcormick Observatory of the University of Virginia, also recorded no detectable wobble. Neither Harrington or Fredrick are willing to toally discount the existence of planetary bodies, but on the other hand, are extremely pessimistic. Harrington, however, is quick to add that in 1977 there was some wobble detected in the North-South direction. In 1985, a few years after van de Kamp published his definitive and last paper on the subject , a Govert Schilling from Utrecht, The Netherlands published the account of an interview with van de Kamp. In essence van de Kamp admonished his critics by stating this his study of Barnard's Star was longer (over 40 years), the number of photographic plates he studied was larger (tens of thousands of plates), he did everything he could to eliminate errors, and yes, he is still under the opinion from his observations that there are indeed two planetary companions revolving around Barnard's Star and that the masses of these planets are 0.7 and 0.5 times Jupiter's mass. To his critics van de Kamp suggests that they (present researchers) spend the same amount of time with the same amount of plates and then, after that, he (van de Kamp) would be happy to talk with them. (Peter van de Kamp died in 1995. I do not know whether he and his critics ever got together to discuss his life's work.)
As a footnote to the above account, it should be noted that Gatewood, using astronometric techniques is trying to ascertain what planetary bodies would be most unlikely to be revolving around Barnard's Star. In a 1995 paper Gatewood suggested that brown dwarfs (more massive than Jupiter by greater than 10 masses, but not massive enough to glow as a star) could not exist around Barnard's Star. In addition, he feels that planets having a mass smaller than Jupiter's may possibly be present.
Although work on Barnard's Star has spanned well over a half century, the jury still seems to be out. No definitive confirmation by the astronomical community as a whole has been established. Only time will tell as to whether a planetary body or bodies orbit Barnard's Star as suggested by van de Kamp.
The visual detection of a planet orbiting a nearby star would be nearly impossible due to the fact that the parent star is millions of times more luminous than the chunk of rock circling it. This is because stars generate their light (and enormous energy) by a process called thermonuclear fusion. The planets on the other hand do not produce any light; they are visible only by reflected light from the stars that they orbit. So how do we detect planets around other stars that are trillions of miles (light years) away?
As it turns out not even the largest Earth based telescope could detect such planets visually even orbiting the nearest star. The methods astronomers have used is based upon gravitation. All objects in the universe exert a gravitational influence based upon their mass. If an object such as a planet had a large enough mass it would exert a noticeable "pull" on its parent star. The planet would be revolving in some sort of orbit around the parent star, just as the Earth revolves around the Sun. Since the system (star and planet) arc moving around (he galaxy this "pull" effect can be detected only after a period of years by examining the positions of the star. The star and planet do not exactly revolve around each other, but rather they both revolve around a point known as the center of mass. The center of mass moves in a straight line as the star and planet revolve around it. So it would appear that the star is "wobbling" by. To illustrate this consider a golf club. The balance point is not exactly at the club head or at the grip. Rather the balance point is somewhere in between the two, closer to the club head, since that's where most of the weight (mass) is. So if you picked up a golf club and tried to balance it (by the way folks, this experiment also works with brooms) it would balance at the point known as the center of mass. The same holds true for the star and planet. The center of mass will be closer to the more massive star than the small minute size planet. In order to detect the minute displacements or "wobbles" that a planetary companion would exert on its parent star astronomers have studied the positions of the suspect stars from a series of photographic plates taken over decades. One such star under scrutiny for many years was "Barnard's star" discovered in 1916 by the great observer E.E. Barnard due to its huge yearly motion across the sky (over 10"/year). Subsequent investigations showed Barnard's star to be the Sun's 2nd nearest neighbor superseded only by Alpha Centauri system in the Southern hemisphere. Its trigonometric parallax was observed to be 0.552", implying a distance of just 1.81 parsecs (5.95 light years).
In the early 1960's a Pennsylvania astronomer, Peter van de Kamp, published his results of Barnard's star based upon a study of over 2400 photographic plates covering the time interval 1916-1962 using the Sproul Observatory 24 inch refractor. van de Kamp in 1968 concluded that Barnard's star had an unseen companion based upon a detected "wobbling" motion. This "wobbling" according to van de Kamp could be explained by a "planet like" companion of about 0.0016 M¤, or 1.7 Jupiter masses. He concluded based upon his data that the planetary companion revolved around Barnard's star every 25 years.
In the Figure above, the dots and average line show the motion of Barnard's star from van de Kamp's data. Most of his pertubation appears to around 1950, presumably from an unseen companion, in which van de Kamp estimates as 1.7 Jupiter masses.
One of the problems in trying to detect the wobbling motion in the Barnard's star example is the problem of errors. In this case the wobbling motion reported by van de Kamp was so small it was perhaps hidden within the threshold accuracy of the measurements. This means that the displacements of the star (caused by the companion's gravitational influence) were on the verge of detect-ability by the equipment used at the time. Also, in 1949 the 24 inch objective of the Sproul telescope was removed from the tube for cleaning! This was perhaps a fatal mistake since astrometric research programs usually span decades, one of the most important things is the stability of the telescope and its objective lens. Coincidentally the major perturbation in van de Kamp's data occurred around 1950. Even more serious was the fact that van de Kamp used a mathematical method of data reduction that involved one approximation after another due to the tremendous amount of labor needed to carry out all of the calculations. In van de Kamp's era of the 1950’s and 1960's computers were in their infancy and not readily available. So in the calculations every single comer was cut to make the task easier, as these computations were done mostly by hand.
In the 1970's a graduate student named George Gatewood, decided to reinvestigate Barnard's star and van de Kamp's claim of the existence of a planetary companion. He worked closely with the great astronomer Heinrich Eichhorn from me University of South Florida. Together these two astronomers used another series of photographic plates taken of Barnard's star over the time interval 1916-1971 from two telescopes: The 20" refractor at Van Vleck Observatory and the 30" Thaw refractor at Allegheny Observatory at the University of Pittsburgh. Computers were now readily available and the task was undertaken by the large mainframe IBM computer located at the University of South Florida. Much earlier in 1960, Dr. Eichhorn proposed a rigorous method to compute and reduce data from star positions that involved a huge system of equations. So huge was the system of equations that everybody stayed away from it due to the fact that it would take a several lifetimes of hand calculations to complete, since computers were not yet available. The technique involved using overlapping plates of the same star field to produce a unique position of the star with much more precision than taking 'averages' of several plates of the same area. When computers finally evolved in the early 1970's Dr. Eichhorn had only one thing on his mind: to challenge van de Kamp's claims of Barnard's star.
And so he did.
In 1973 Gatewood and Eichhorn published their results in a landmark study that shook the astronomical world. Their comprehensive rigorous painstaking study of all of the photographic plates that existed of Barnard's star failed to turn up any of the "wobbling" motion that van de Kamp claimed to have detected. The reason was obvious to Gatewood and Eichhorn. van de Kamp's methods of cutting comers in the data reductions is no longer necessary since the advent of large powerful electronic computers. Eichhorn's 1960 rigorous method of computing the star positions was superior than van de Kamp's approximate methods. From their 1973 paper, they wrote “Thus we conclude, with disappointment, that our observations fail to confirm the existence of a planetary companion of Barnard’s Star.” (Peter van d Kamp was the referee for this paper before it was published!!)
In the Figure above, Gatewood and Eichhorn's motion of Barnard's star is plotted as points, with the size of the points in proportion to their weight. The dashed line represents van de Kamps' published orbit, and the straight line represents the motion of an unperturbed star. (star with no companion).
More recently Dr. Galewood has developed a new approach toward achieving high accuracy in star positions. The Multichannel Astromelric Photometer (MAP) as it came to be known consisted of a telescope feeding starlight to a series of parallel lines (Ronchi rulings) in (he focal plane. The light would be picked up by several photomultipliers. Instantaneous recording of the photomultipliers output gives the east-west spacing of a target star, with respect to adjacent field stars to very high accuracy. By moving (the Ronclii ruling across the target star's light in 4 perpendicular directions, hundreds of sinusoidal fluctuations occur and are recorded. The distances between stars is then computed by comparison of the phase of each star's signal to the rest. Unprecedented accuracy of a star's relative position can be obtained in just a single night of observations. According to Gatewood, accuracies of 0.003 arc second can be achieved in just 2 hours! Previously such accuracies could only be obtained after combining data from 100 exposures taken over a full year on photographic plates. With such precision available so quickly, a sub-milliarcsecond era of astrometry has begun. Nearby objects can be studied in more detail and the radius of objects within the solar neighborhood can be expanded greatly, thus increasing our sample of objects with accurate distances. Relative parallaxes will be known routinely to 0.001 arc second thus pushing the distance we can see accurately to nearly 100 parsecs. Needless to say the wobbling motion of stars with planetary systems will be easier to detect for the suspects in the local solar neighborhood.
REFERENCES
Gatewood, G., Eichhorn, H., 1973, An Unsuccessful Search for a Planetary Companion of Barnard’s Star (BD + 4° 3561), Astronomical Journal, 78, p. 769.
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